There is increasing pressure for grain growers to present their produce onto domestic and international markets in good condition, free from live insect pests and free from chemical pesticides. The marketing requirement that the grain must be free from chemical pesticides arises for two reasons. Firstly, insects inevitably become resistant to chemicals hence increasing dosages and/or new pesticides must continue to be developed. Secondly, consumers are becoming increasingly intolerant of chemicals being admixed with foodstuffs. As a result of these pressures, engineering, as opposed to chemical solutions, are sought to the problems of grain storage.
In Australia, grains are harvested quite warm, typically with a temperature of 30.degree. C., although this is quite variable. Such temperatures at harvest time are conducive to the rapid growth of insect populations. Further more, they are sufficiently high to cause the rapid degradation of desirable grain properties, such as those required for bread making or for malting barley. One way of reducing these problems is to cool the grains, which confers the following benefits to storers and handlers of grains:
cooling slows or reduces the rate at which insect populations grow; PA1 desirable grain properties such as germination and baking quality are preserved; PA1 in cases in which chemical pesticides are applied to grains, cooling slows their rite of decay, and makes the rate of loss more predictable; PA1 by reducing temperature gradients in the grain bulk, free convection currents are reduced that transport moisture from warm regions of a grain bulk to cooler regions; PA1 by slowing down the rate of fungal activity, cooling allows damp grain to be stored for longer periods of time before it is dried.
A convenient method of cooling bulk stored grains is to force ambient air through the grain, usually during the night when the ambient temperature is low. However, since food grains are hygroscopic they adsorb moisture from the air, and heat is liberated as a result of moisture condensing on the grains. As a consequence, the grains do not generally cool down to the temperature of ambient air, and generally the so-called dwell temperature of the grains is higher than ambient temperature. For example, wheat with an initial moisture content of 11% will cool only to about 27.degree. C. when ventilated with air with a temperature of 20.degree. C. and relative humidity of 90%. The crier the grains, and the more humid the climate the more difficult it is to cool the grains. One way of cooling grains further is to ventilate them with air that has been cooled by a conventional vapour compression air refrigeration unit, typical of those used to air condition buildings. A second method is to ventilate the grains with air that has a temperature close to ambient temperature, but which has been dried or dehumidified. The low humidity air causes a very small proportion of the grain to dry, and this absorbs latent heat of vaporisation which causes the grain bulk to cool. For example, wheat with an initial moisture content of 11% will cool to about 16.degree. C. when ventilated with air that has a temperature of 20.degree. C. and a relative humidity of 20%.
Desiccants have been used to condition air that is subsequently used to cool buildings during the day. Such systems often manifest themselves as rotating dehumidifying wheels in which the desiccant in one half of the wheel is used to dry air, whilst the desiccant in the other half of the rotating wheel is being regenerated. After the air has been dried it is usually cooled by injecting water sprays into it. This air is unsuitable for cooling grains because its enthalpy is little changed by its being cooled, and its high relative humidity prevents the stored grains from cooling, as described above.
An open-cycle desiccant bed system for cooling food grains has been described by Ismail, Angus and Thorpe (Ismail, M. Z., Angus, D. E. and Thorpe, G. R. (1991) The performance of a solar regenerated desiccant bed grain cooling system. Solar Energy Journal, 46(2), pp 63-70). In this system, ambient air is dried during the night in two separated desiccant beds arranged in series in the air flow, and the heat of sorption is reduced by natural convection in a multistage process by passing the air over heat exchanger means after it exits from each bed of desiccant. During the day, the heat exchangers act as solar collectors and they are used to heat ambient air that is then used to regenerate the desiccant. This device reduces the humidity of ambient air but the air flow rate is limited to about 4 liters per second per square meter of heat exchanger area. One reason for this is that the overall heat transfer coefficient from the heat exchanger surfaces is low. As a result, it is difficult to dissipate the heat of sorption to the atmosphere. Furthermore, the desiccant beds need to be quite deep to ensure the velocity of the air that flows through them is sufficiently low to maintain a low pressure drop across the device. This large depth limits the rate at which heat can be dissipated from the desiccant beds, and as a result, the beds do not operate isothermally.